Make the Lemon Curd:
Heat a few inches of water in a saucepan over medium heat, bringing it to a simmer.
In a non-metal heatproof bowl, whisk together the lemon juice, eggs, egg yolk, granulated sugar, and salt using a non-metal whisk.
Place the bowl over the saucepan, ensuring the steam from the simmering water gently heats the mixture. Continuously whisk as the temperature rises and the mixture thickens.
Once the mixture reaches 180°F (82°C), remove the bowl from heat and continue whisking to help it cool down.
When the temperature lowers to 135°F (57°C), add the room temperature butter in ½-inch pieces, blending with an immersion blender until the butter is fully incorporated.
Make the Whipped Mascarpone Cream:
In the bowl of a stand mixer fitted with a whisk attachment, add the room temperature mascarpone and heavy whipping cream.
Whisk on medium-high speed until stiff peaks form. This should take only a short amount of time.
Assemble the Tart:
Spread the whipped mascarpone cream evenly over the tart shell.
Pour the lemon curd over the mascarpone cream, spreading it evenly.
Chill the tart in the refrigerator for 1-2 hours.
Before serving, garnish the tart with fresh raspberries, blackberries, and a sprinkle of fresh thyme.